A lack of transparency combined with a reputation (earned or not) for guiding primaries to their desired outcome, it's not hard to see why some people might think this is an example of the DNC putting their finger on the scale in some way. There's no evidence either way, which means confidently claiming incompetence is fairly baseless, as well.

If we want to talk what things are generally true, sure, I'd put money on incompetence without any context, but there IS context to this.